The present invention relates to heterogeneous catalysts that are useful for coupling carbon-containing compounds.
Homogenous catalysts containing solubilizing ligands have been used for coupling carbon-containing compounds and generally have high catalytic activity and give high yields of the desired products. Removal of the catalyst after its use in the reaction can however be quite difficult, especially on an industrial scale. The ligands of the homogeneous catalyst can dissociate from the catalyst and can be difficult to remove, thereby creating purification problems. Laborious workups, scavengers, and crystallizations must therefore be used to remove and/or reduce the presence of the catalyst and/or free ligands to acceptable levels.
In an effort to avoid the use of homogeneous catalysts, heterogeneous catalysts have been utilized to couple carbon-containing compounds. A variety of heterogeneous catalysts is known in the art and includes metals on solid supports. Heterogeneous catalysts are usually inexpensive, commercially available, and exhibit minimal leaching of the metal from the solid support, thus simplifying purification. Examples of commercially available heterogeneous catalysts include palladium metal on silica gel, activated carbon, and polymers.
However, some of these catalysts are expensive and often not available in bulk quantities. Further, palladium on carbon supports, including activated carbon, can be difficult to use since the reaction mixtures tend to turn black, thereby obscuring any reaction changes that can occur during the reaction. It is also difficult to remove the carbon support from the equipment utilized to perform the reaction.
What is needed in the art are alternative homogenous catalysts for coupling carbon-containing compounds that can be easily removed from the reaction mixture.